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How to compare them honestly
Judge a username tool on four axes: platform coverage, detection quality, output, and price model. Coverage is the easiest to advertise and the least interesting on its own — a thousand stale rules produce a thousand guesses.
Detection quality means how it decides a hit. A tool that treats HTTP 200 as proof of existence will report a match on every site that serves a soft 404 page. A tool that reads page text or uses platform-specific rules does considerably better.
The open-source checkers
Sherlock remains the reference implementation: a compact site list, parallel requests, plain output. It is fast, readable and trivially extensible, which is why it is still the first name people learn.
Maigret goes wider with thousands of sites and more metadata, and adds report formats. WhatsMyName contributes a curated, community-maintained list of URL patterns and detection hints that other tools consume.
The shared weakness is maintenance cost. Sites change markup constantly, so volunteer rule sets drift. Expect to fix failures yourself if you rely on them seriously.
Hosted searches and people-search sites
Hosted tools trade maintenance for convenience: you get a report and a link, and your queries go through someone else's infrastructure. For routine self-audits that trade is fine.
People-search subscriptions are a different product entirely. They sell aggregated records — addresses, phone numbers, relatives — not username-level presence, and their data freshness varies enough that you should verify anything decisive through a primary source.
What matters for the case in front of you
For a self-audit, a fast local checker and thirty minutes of reading beats a premium subscription. For research where the output has to stand up to scrutiny, the tooling matters less than whether you recorded what each page actually showed.
Whatever you choose, keep the manual step. Open the profiles yourself. The tool finds candidates; only you can decide what a page means.